Support HTML emails from mimemail

There are 3 levels of emailing in Drupal:
- email build (this module alter it);
- email format (`mimemail` module, reroute_email module isn't aware of the formatting);
- email send.

If `mimemail` module is used, we can find UI to configure email build/format on `mailsystem` (required by `mimemail`) configuration page:
`/admin/config/system/mailsystem`

Rerouting is implemented on the email build level (`hook_mail_alter`) and on this stage we have no information about the email formatting (it can be configured in different ways for each email).

There is `Email format` setting in `mimemail` configuration form (`/admin/config/system/mimemail`), which says:
> If you are using Mime Mail as default mail system, make sure to enable "Convert line breaks into HTML".

Once we add `Convert line breaks into HTML` filter to the configured `Email format` (`/admin/config/content/formats`), everything works as expected.

In other words, while using `mimemail` module as email formatted, it (email formatting) should be configured via it.

I have installed:
- new Drupal 10 instance + mimemail + reroute_email;
- add `Convert line breaks into HTML` filter to `Email format` used by `mimemail`;
- sent a test email via test form (`admin/config/development/reroute_email/test`);
- sent a core email by resetting the password (`/user/password`);
Both emails look correct.

At this moment, it looks like there is nothing to change in this module.
Let me know if this message helps!

Automatically closed - issue fixed for 2 weeks with no activity.